Because the copper clad laminates used in electronic and electrical devices are subject to strict safety requirements to prevent and retard fires, epoxy resin compositions with introduced bromine and other halogen atoms are widely used for purposes of flame retardancy. It is relatively easy to increase the bromine content of epoxy resin, and a high glass transition temperature, heat resistance and other necessary properties of laminates can be achieved by combining a highly-brominated epoxy resin with a non-brominated epoxy resin or less-brominated epoxy resin.
For environmental reasons, epoxy resin compositions having phosphorus atoms introduced into the epoxy resin instead of bromine and other halogen atoms have become popular in recent years. A technique has also been proposed for introducing phosphorus atoms into the curing agent (Patent Document 1).
In recent years, however, signal transmission speeds have increased in the telecommunications and other fields, and when signal frequencies are increased as a result there is generally more transmission loss, which interferes with device functions. It has therefore become necessary to minimize transmission loss by using materials with low dielectric constants and dielectric tangents in the multilayer boards of printed wiring boards.
However, epoxy resin compositions having introduced phosphorus atoms in place of halogen atoms tend to have poor dielectric properties, with higher dielectric constants and dielectric tangents.
Two techniques that have been proposed for improving the dielectric properties of epoxy resin compositions and the like are to use a polyphenylene ether having hydroxyl groups as the curing agent (Patent Documents 2, 3), and to use an epoxy resin comprising a glycidylated polyphenylene ether having hydroxyl groups (Patent Document 4).    Patent Document 1: Japanese Translation of PCT Application No. 2008-501063    Patent Document 2: Japanese Patent Application Laid-open No. 2004-231728    Patent Document 3: Japanese Patent Application Laid-open No. 2004-217854    Patent Document 4: Japanese Patent Publication No. 3879831
However, when the curing agent is a polyphenylene ether having hydroxyl groups, adhesiveness is poor because of the polyphenylene ether structure, and poor interlayer adhesiveness has been a problem with these epoxy resin compositions. Another problem has been incompatibility in epoxy resin compositions containing phosphorus atoms.